Today I assembled the regulated tank block I got from Travis back in the summer.... It went well, and after a short learning curve installing shims, I was able to adjust it from 1000 psi up to over 2000.... I didn't try and go higher, but I am sure it will.... It has a 3K burst disc on the output side, and I didn't want to push it.... The adjustment is quite sensitive, you don't have to turn the 10-32 adjusting setscrew very far to move the output by 100 psi.... maybe 1 flat on the allen key or less....
Since the Cothran valve cannot be adjusted using hammer preload (or SSG gap), and needs to have the pressure adjusted to change the velocity, or retune for a different pellet weight.... it was a perfect match for an externally adjustable regulator.... Since there are two excellent pellets made by JSB that are quite different in weight (25.4 gr. and 34.1 gr.) I decided that to install the .25 cal upper on the lower that used the Cothran valve and the adjustable regulated tank block and I installed a 500 cc, 3000 psi bottle on it.... My initial adjustment of 2000 psi drove the 34.1 gr. JSB King Heavies at over 980 fps, but I used the opportunity to find out where the cliff was, and where the SSG gap needed to be adjusted to be close to the cliff, but solidly on the plateau, where the valve is operating stably.... That was with 2 turns of gap.... Then I shot the tank down to about 1700 psi so that I could drop the setpoint adjustment....
When increasing the setpoint pressure, you back out the 10-32 screw, which lowers the HP seat, moving it away from the piston and Belleville stack.... The seat opens, allowing more air to flow to the output side, raising the pressure and pushing the piston towards the (now lower) seat, against the Bellevilles, compressing them until the seat closes at the now higher pressure.... This puts no additional load on the seat, and can be done at any time.... When you want to decrease the output pressure, however, you need to drop the tank pressure below not only your current setpoint, but also below where you want the new setpoint to be.... The reduced pressure on the piston allows it to move away from the seat, and this allows you to turn the 10-32 screw in without putting additional pressure on the seat and possibly damaging it.... I wanted to start at 1800 psi and work my way back up, so I drained the tank down to 1700 and turned the 10-32 adjuster in until I felt slight additional resistance when the seat contacted the piston.... If you do this carefully and slowly, you can feel when they make contact.... When I added more air to the bottle, the gauge on the output side of the tank block moved up slightly, to about 1800 psi.... I shot a few shots, and the velocity was down to about 920 fps, so I simply backed the 10-32 adjusting screw out a fraction at a time, shooting a couple of shots at each setting, until I got my target velocity of 950 fps.... I then shot a couple of shots at each SSG gap setting, and recorded the results, which are on the graph below in red....
You can see the typical "cliff" exhibited by the Cothran valve which starts just before 3 turns out.... The velocity at 3 turns out is very unstable, with an ES of nearly 200 fps, because the valve is about to fall over the cliff.... However, at 2 turns of gap, it is rock solid, with about a 1% ES.... From previous experience I know that is the optimum place to run a Cothran valve, with the lowest hammer strike that allows stable operation and low ES.... I tried the 25.4 gr. pellets, and the velocity was over 1050 fps, so to get them to work I knew I had to lower the setpoint pressure.... I started simply shooting the 25.4 gr. pellets and watched the velocity fall as the pressure dropped.... I was pretty shocked to discover that to get down to 950 fps I had to drop the pressure in the tank all the way down to 1400 psi.... I shot a few more shots, to drop the tank pressure to 1300, and then turned in the 10-32 adjusting screw until I could feel it contact the seat.... I added air to the tank, the output gauge barely budged, and the velocity settled in at about 910 fps at about 1300 psi.... I slowly backed out the pressure adjusting screw, shooting a couple of shot at each adjustment, and stopped when the velocity with the 25.4 gr pellet reached my target of 950 fps.... The gauge indicated 1400 psi.... and I confirmed that by shooting the tank down until the velocity just started to drop, while the tank was still connected to the accurate gauge on my Great White.... Yes, I was getting 51 FPE at only 1400 psi.... I ran through the gap adjustments, and recorded the other curve (in blue) on the graph above.... The optimum gap was at 3 turns out, and the cliff started at 4 turns out, 1 turn more than at 1900 psi.... As expected, the valve requires a bit less hammer strike at 1400 psi than at 1900.... I also shot a few of the 34.1 gr Heavy pellets at that same setting, and that velocity is shown on the graph above as the black dot (872 fps).... So, with the 34.1 gr. pellets, I was getting 57.6 FPE at only 1400 psi.... pretty remarkable, and no adjustment made from the 950 fps setting with the 25.4 gr Kings....
Now that I knew that 3 turns out was the optimum setting for the 25.4 gr. Kings at 1400 psi, I recorded the pressure in the 500 cc bottle (using the gauge on my Great White), and shot 8 shots, on regulator, at 1400 psi.... I averaged the velocity (952 fps = 51.1 FPE), and recorded the pressure drop, which was 200 psi.... That works out to 0.97 FPE/CI, which is pretty low, but we are in new territory here, getting 51 FPE at only 1400 psi.... I repeated the test using the 34.1 gr. Heavies, and as I expected, the pressure drop was exactly the same, at 200 psi.... Since the energy per shot increased to 57.6 FPE, however, the efficiency increased to 1.10 FPE/CI.... I then backed out the 10-32 pressure adjuster to increase the setpoint back to 1900, set the SSG gap at 2 turns, and shot another 8-shot magazine at that setting with the 34.1 gr. Heavies.... The pressure drop was just over 200 psi, so I recorded it as 210 psi, and with the average velocity of 950 fps (68.4 FPE), that works out to an efficiency of 1.24 FPE/CI.... pretty decent at that power level....
The pressure drop per shot for all these tunes runs just about 25-26 psi per shot from the 500 cc bottle.... If we use a fill pressure of 3000 psi, with the 1900 psi setpoint, that should give 42 shots at over 68 FPE with the 34.1 gr Heavies.... and with the 1400 psi setpoint, that should give 64 shots at 51 FPE with the 25.4 gr. Kings and nearly 58 FPE with the Heavies.... I think those are quite acceptable numbers, and tuning back and forth between the 25.4 gr. and 34.1 gr. pellets (both at 950 fps) requires only resetting the external regulator adjustment, and one turn on the SSG gap.... In fact, if you don't care about getting the full 64 shots at the 1400 psi setting, you don't even have to change the SSG gap, you can leave it at 2 turns, and just lose a few shots.... If you have the gun tuned at 1400 psi, you can shoot the Kings at 950 fps and the Heavies at 870 fps, for the same number of shots, with NO adjustment necessary....
Bob